The gas sensor device and the fuel cell device have functions to bring a gas into contact with the ceramic members that constitute these devices, so as to have particular component of the gas adsorbed onto the ceramic surface or permeate therethrough. To provide such a function, such attempts have been made as forming metal layers (electrodes) in a highly porous structure so as to increase gas permeability, or controlling the surface area covered by an electrode by forming the electrode in comb or other shape in case the electrode is dense (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-317555 and Japanese Unexamined Patent Publication (Kokai) No. 6-258281).
A metal layer having a high void ratio can be manufactured by, for example, forming an electrode by printing a metal that can be sintered only at temperatures higher than the sintering temperature of ceramics, and firing the ceramics and the electrode metal at the same time.
There is also such a method as an electrode paste prepared by mixing a metal powder and a ceramic powder that cannot be sintered at the sintering temperature of the electrode metal is printed and fired. Other methods of forming a metal layer having high void ratio include firing of Pt electrode described in Japanese Unexamined Patent Publication (Kokai) No. 11-51899.